I finally got time to check my calculation again, and found the error, third line on the 2nd page. It's 63*R*X on the left side, instead od 33*R*X, so that really gives a mis-alignment of -8*Rs, what I already found for the other configuration also, and what e61_phil found by means of his SPICE calculus.
Next step was to measure this Rs. On my 15 year old precision grade ELMA switch, all measured contacts (still) showed 5mOhm only, with a slight variation by switching actuation of about +/- 1mOhm.
These 5mOhm therefore give about -40mOhm adjustment error or ~ 0.16ppm. (with 100 * 25kOhm resistors)
That's still good enough, but obviously can be compensated, by the remark in the Fluke manual "... adjustment of the interconnection resistances.."
This switch resistance variation gives +/- 8 mOhm, or an estimate of +/- 0.032ppm, which might be the root cause for these 0.042ppm worst case, Fluke mentioned.
Maybe they also include in this parameter a permanent increase/ageing of the switch contacts.
All three parameters, the low nominal switch contact, its variation over actuation, and its deterioration, I would expect inside the specification of their custom made switches.
Third step was to compare my findings on my box with the 752A schematics, and to identify this compensation / adjustment of the -8*Rs, and of further cable / switch resistances.
I have to admit, that I didn't have the schematics available 15 years ago, and I did not check this paragraph 3-30 carefully enough.
Long story short, this compensation is done very obviously by a strange long cable, denoted as: '36.75 inches long AWG 22 solid Cu'.
That exact specification gives 49.4 mOhm, or 0.12 ppm compensation.
This cable-resistor is connected in series with the upper 360 kOhm leg of the divider, but only in the unfolded, divider mode.
So that's the way, how to compensate these said effects.
The value of this resistance also has the correct order of magnitude, so one can conclude, that the switches of the 752A also have about 5 mOhm contact resistance.
Fluke obviously made a precise analysis and summation of all involved cable lengths and number of switch contacts and then determined this specific length of this ominous cable.
It should be located between both switches, coiled up, although I think there is another failure in the schematic, as this compensation is not effective in the 100:1 divider.
Anyhow, I think that I now have fully understood this aspect. Sort of black magic inside the 752A, which is engineered very well, over all.
I also think, that it's possible to replicate its high accuracy using volt-nuts grade facilities.
Frank